Treat import statements with only inline type as weak

[denis-sokolov]

A statement like one below today is treated as a strong import by ts-helper, and triggers cycle detection.

import { type Foo } from "./foo";

However, TypeScript compilation omits the entire statement and as such it is equivalent to this one that does not trigger cycle detection:

import type { Foo } from "./foo";

If you accept this PR, you might also want to adjust the section right below that handles the exports:

https://github.com/quicktype/ts-helper/blob/0ca5a0987c22f03c3c61a8301cf20ca381191efb/src/index.ts#L264-L269